Field of the Invention
Embodiments of the present invention generally relate to a micro-electromechanical system (MEMS) variable capacitor.
Description of the Related Art
The most frequent way of actuating a MEMS variable capacitor is through the use of electrostatic force. Generally, the voltages required for actuation are in the order of 10V to 100V. Using these voltages imply that the electric fields become very large in the sub-micrometer dimensions used. The electric fields are expressed by the equation E=V/gap where ‘V’ represents the voltage between two electrodes and ‘gap’ is the distance between them. Properties of dielectric materials change under high electric fields (e.g., charging), and in some cases there can be physical damage on the dielectrics. These factors reduce the lifespan, performance and operating conditions of the MEMS digital variable capacitors.
Therefore, there is a need in the art to reduce the magnitude of the electric field in a MEMS variable capacitor while maintaining a high capacitance state when the MEMS is landed.
Additionally, inter-modulation performance for RF-MEMS variable capacitors is a difficult specification to meet. This is because the MEMS devices contain movable parts that provide the tuning of the capacitance. RF voltages between the two electrodes attract the two plates of the MEMS variable capacitor further together if there is any remaining gap, forcing a further capacitance change.
Depending on the MEMS device electromechanical design and on the intended application inter-modulation specifications, the maximum allowed movement can be as small as few nanometers for an applied voltage between the two plates of the variable capacitor of several volts.
Even surface treatments such as chemical mechanical polish (CMP) can be insufficient for good inter-modulation performance. For example, after chemical mechanical polishing (CMP), some bumps can appear at the metal-dielectric interface. These bumps can have dramatic effects on the RF performance as they create extra gaps where the membrane can deflect. Also the oxide surface itself after CMP can sit elevated with respect to the metal surface, leading to gaps between the membrane and the electrodes. Additionally, the CMP bumps act as electric field intensifiers which again lower the reliability of the part.
Therefore, there is a need in the art for a solution to address the possible inter-modulation issues while keeping the electric fields low enough for long reliable operation.